LM317 voltage regulator keeps dying.

@davee OOPS, when I say not common, I mean no wire running from NEG1 to NEG2 to NEG3, each NEG is connected to ground/chassis. The point of that is to guarantee that if 110VAC ever comes into contact with the case of the PSU, it blows a breaker somewhere. Similarly, if some piece of equipment attached to any of the NEGs has a fault that puts a voltage on the wire between it and the PSU NEG then again, it blows a breaker.

I am sorry, I do not understand point 1, point 2 probably, 3 maybe, 4 very likely.

Out of an abundance of caution, I will make you a promise that the entire workbench will be fed from a GFCI so any leakage is detected and power is shut off.

Dave, I was an industrial electrician before I got into IBM. I worked on MG sets over 12 ft tall. Before working on one of them, we had to remove all metal from our body and only take whatever tool was needed after a co-worker wrote it on a check-in sheet. On smaller stuff, my high-voltage Voltmeter probes replaced my fingers. I often checked that green wire to steel case connection because I was paranoid. But in this case, I am simply not getting it. I think I drew you a diagram that fairly represents what I plan, so if you can draw a path for the current that is dangerous to me or my equipment, then show me but it's very simple and straightforward.

Dave, I respect your expertise and am happy to receive your concerns, but I don't understand and will have to learn the hard way. Thank the mighty OZ for clamp meters.

Hi Ron @zander,

   Assuming you are following Bill's design, this is a unit with 24 Vdc max, not including the AC input to the transformer.

If it has a metal case, then I assume you will ensure this is earthed, in case of a breakdown of insulation on the incoming mains feed or in the transformer primary to the metalwork.

So, given the above, plus your general experience of mains (and above) voltage wiring, I am not generally concerned about safety in this case ... I trust that you will deal with this adequately.

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It is purely about how you expand Bill's design up to (say) 5  independently votage controlled and current measured outputs of voltages in the range 2 to 20V ... which is not (generally) dangerous, but could be annoying if it doesn't work properly, after you have put a lot of work into it.

It could also destroy some electronics if the voltage exceeds the expected recipient's range, though that is usually more of a danger to your wallet than your health.

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As for the points .. same numbering

1/ (I do not understand) Are you connecting Mains earth to any part of the circuit supplied by the transformer's secondary? Bill connected it to his NEG output.

A plausible, but 'not recommended' point could be the negative terminal of the smoothing capacitor.

Leaving the whole secondary circuit 'floating' is feasible, though not generally recommended. If so, I think the transformer should have extra protection and/or shielding between the primary and secondary as precautions against internal insulation breakdown between the windings.

2/ (probably) If you power the same unit with more than 1 output, then this means there will be more than 1 NEG output .. how do you intend to connect to a 'single' unit with only one 0V line?

3/ (maybe) Basically the same problem as 2 ... networks like I2C require both ends of the network to be at the same earth potential.

4/ (very likely) Most mains powered computers will connect to mains earth .. and this includes the USB cable. This would give a problem if PSU circuit is referenced to any mains earth at any point, other than the specific NEG output used to power the unit.

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Depending on current levels, any of these situations may or may not cause the unit(s) powered by the PSU to not work correctly, depending how resilient they are to power supply variations, etc. which means it is yet another of these cases where any prediction of failure or success is more luck than judgement.

The only obvious personal safety concern could be addressed by a using a transformer with secondary windings specifically provided with extra protection in case of an internal failure. (I think this is a bit of a specialist subject.)

The other concerns are more likely to give rise to things like data errors than direct physical damage. The 'confused' positive and return paths when more than 1 output is used could result in transitory voltages above the expected value, and in turn could damage components.

Sorry, I can't give absolute success or failure ... good electronics design aims to leave generous margins to be resilient to modified voltages, noise, etc. ... your design threatens to eat into those margins ... but there are too many choices and unknowns to predict the exact consequences.

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Best wishes, Dave

@davee Dave, I appreciate your concern for my safety, but between your UK terminology and my problem with reading new technical information, all I am hearing in my head is noise, my problem not yours. I have an existing PSU and routinely use multiple voltages. At this point, there must be something I am not telling you or explaining wrong because I am sure everything you say is accurate, but it is not registering with me.

I built my own solar system with amperages exceeding 400A using 4/0 AWG (107mm^2) wire as thick as my thumb, and everything is safe and working correctly. Trust me when I tell you doing electrical work on an RV that is a metal house sitting on insulators (rubber tires) is tricky. To give you an idea of how seriously I take safety, I purchased two different power management devices. One would have done the job, but the 2nd one is my secret weapon for underpowered RV parks and costs almost $1,000. The other device has the Emergency Power Off with Bluetooth access, I can see my input voltage (pic enclosed for FYI)

When I get it built, I will test it as best I can, and then send you pictures, and maybe even before then, I will at least have a neat diagram (hard to do in an RV sitting in my easy chair).

Let's move on, I am many months away from starting this project.

I truly appreciate your concern, but I can not see anything to worry about as everything will be grounded and connected to what we colonists call a GFCI, not sure what your terminology is but I know it is different.

EDIT Oops forgot the pic

Hi Ron @zander,

  I do not doubt your ability to build power systems using well designed units, etc.

I will 'bet' that none of the systems tries to measure the current flow in the return line (excepting RCDs which compare outgoing and returning current, with the aim of tripping when the two currents do not match). And furthermore, they will not be measuring current by inserting a shunt resistor into the circuit which increases the potential 'weirdness' when it is in the return line.

 So I suspect this may have been the first time you have been faced with a resistive (shunt) current measuring system that is on the return side ... frankly it is a cheapskate trick to make current measurement devices for a dollar or so, as by having it on the return side means that it only has to deals with low voltages ... in fact much less than 1 V, that is easily amplified and directly fed to a analogue converter. Measuring the current on the 'high side' is much more difficult with a shunt approach ... other methods like Hall effect and (for AC) current transformers are much more straightforward to implement, but also more expensive than $1.

If you have a simple 1 output system, then arguably it is a good way of saving money, complexity, etc., and Bill's PSU is a good example of just that.

However, when you try to scale to have two or more outputs, it starts to get much more messy.

My only 'real safety' worries are that there isn't a straightforward point to connect to the mains earth, which as Bill's circuit shows, is straightforward with just one output:

After that, there are just so many ways the shunts can accidentally get paralled, etc., depending on what you do, which are unlikely to be a safety hazard, but could cause all sorts of weird effects to both the powered circuit operations and also the readings on the meters.

Just trying to be helpful.

Best wishes and take care my friend, Dave

@davee I know you are just trying to be helpful, and when I get around to building this I will be extra careful. I know a bit about shunts, I have a 500A shunt connected directly to the 7,200W battery bank's most negative terminal.

Hi Ron @zander,

I have a 500A shunt connected directly to the 7,200W battery bank's most negative terminal.

Which is ok if there is only possible energy source that can feed through that shunt ... If you had two sources that could both drive current through the same shunt, it can get messy.

Best wishes, Dave

@davee Yes, when helping other RVers build a solar setup, I always go to great lengths to ensure they understand that the shunt is the ONLY thing connected to the battery negative, and ALL other negative feeds and loads attach to the other end. This way, we know exactly how many AH are stored in the battery. 

As far as my multi meter setup I do not violate the principle of one shunt one feed and a diode placed in the down leg guarantees it, heck, I could even put a diode in the positive feed into the VR now nothing can go anywhere but where it is supposed to and I don't really believe I need the diodes.

@davee I am NOT going to make a multiple outputs PSU, your concerns are taken very seriously by me, I just wish I understood why. I will stick with Bill's design. I changed my mind about trying to build and test when I realized I will have a total of 3 PSU's so I think that will cover all my normal use cases. I think using the linear supply for 3.3V, the switching supply for 5V and the ATX for 12V will handle all or nearly all requirements, and the switching supply can handle multiple outputs as it only has the one metered output, and I have already used it that way. Now I have 4 extra meters. I do appreciate your advice even if I seem to be stubborn or stupid, it's just that I want to understand and I will challenge and test you in order to do so. One of my favourite sayings is, 'out of the hottest fire comes the strongest steel'. I apologize for taking up so much of your valuable time, but I do appreciate your concern.

Hi Ron,

   Probably a good decision!  As for what to do with 4 spare volt and current meters....? A challenge for the future perhaps?

Best wishes, Dave